Generally, flake aluminum oxide has a high industrial applicability since it has excellent wear resistance, corrosion resistance, heat radiation property, luster and the like. In particular, due to its excellent property in luster, it has been increasingly popular as a base material for pearlescent pigment.
In order for the flake aluminum oxide to be used as a base material for pearlescent pigment or the like, it should have a high chemical stability, a high aspect ratio and a flat surface. For this conformation, it is necessary to inhibit crystal of the flake aluminum oxide from growing toward a specific direction when the crystal grows.
There are typical methods of manufacturing aluminum oxide such as a Bayer method, a Verneuil method, a Czochralski method, a hydrothermal synthesis method, a molten salt method and the like. An advantage of the Bayer method, the Verneuil method or the Czochralski method is that it is possible to mass-produce aluminum oxide, but it is difficult to control a crystalline formation and a shape of powder to be uniform with these methods. Meanwhile, it is easy to manufacture flake aluminum oxide by the hydrothermal synthesis method and the molten salt method and easy to control a stable crystalline form and a shape of particle, so it has been known that these methods are appropriate for a commercial synthesis of flake aluminum oxide.
On examining related patents about a method of manufacturing of flake aluminum oxide, Japanese Patent Laid-open Publication No. H7-33110 discloses a method of manufacturing hexagonal plate shaped aluminum oxide by adding phosphoric acid ion as a crystal growth inhibitor in a hydrothermal synthesizing apparatus. Further, French Patent Laid-open Publication No. 2441584 discloses a method of manufacturing macro crystalline flake aluminum oxide crystal powder having a diameter of 10 to 200 μm and an aspect ratio of 7 by performing a reaction using aluminum hydroxide as a raw material of alumina and cryolite as flux at a temperature of 1200 to 1450° C. Furthermore, Korean Patent Publication No. 1996-64 discloses a method of synthesizing aluminum oxide gel powder by performing a reaction with AlF3 gas at a high temperature.
However, the above-mentioned typical methods have some problems as follows. In other words, it is possible to synthesize thin flake aluminum oxide at a comparatively low temperature by a hydrothermal synthesis method but because a high-temperature and high-pressure hydrothermal synthesis apparatus is needed for an industrial mass-synthesis, it is a very cost-consuming method, and since the powder synthesized by the hydrothermal synthesis method has a particle size of 20 μm or less, it is not appropriate for a base material for pearlescent pigment.
Furthermore, as for a vapor reaction in which a starting material and an additive are reacted in the vapor state, it is possible to lower a synthesis temperature by 100° C. or more in comparison with a flux method using molten salt. But this method needs an expensive apparatuses for making the additive vaporized and for controlling an appropriate atmosphere for the synthesis and further needs an expensive electric furnace to control sintering under the controlled atmosphere. Therefore, this method is not appropriate for an industrial mass production.
A flux method using a molten salt is a method for growing crystal of the solute in a molten salt molten at a high temperature, wherein the molten salt promotes a synthetic reaction of oxide and accelerate an aeolotropy of a particle and further lower a crystallization temperature by 100 to 200° C. Since the molten salts are very important in synthetically manufacturing flake aluminum oxide by this method, it is essential for selecting an appropriate molten salt. When flake aluminum oxide is synthesized by this flux method, it is possible to synthesize powder in a temperature of 1250 to 1450° C., and to synthesize macro crystalline powder having a particle size of 20 μm or more.
International Publication No. WO 2004/060804 discloses a process of producing flake aluminum oxide by using a molten salt method, specifically by adding molten salts such as metal fluorides or the like and additives. But in case of manufacturing flake aluminum oxide with this method, it is necessary to carry out a heat treatment for a long period of time at a high temperature of 1,250° C. or more and in case of a mass production, it is impossible to perform a uniform heating due to the difference of temperature gradients in a crucible and thus it is hard to obtain flake aluminum oxide having a uniform particle size distribution in which multi-crystal and twin-crystal are 5% or less.
In this connection, the present applicant has disclosed a thesis on “a microwave synthesis of alpha (α) aluminum plate shaped body by a flux method” (Journal of The Korean Ceramic Society, Vol. 39, No. 5, pp. 473-478, 2002) in order to solve the above-mentioned problems. Such a microwave synthesis method uses a frequency of 2.45 GHz, so it is possible to quickly generate heat at an early stage. However, the microwave is penetrated too short to generate the heat within a material itself. Therefore, when a large quantity of a material is heated with such the microwave synthesis method, there may occur differences in temperature between a surface and an inside of the material due to a low penetration depth so when the microwave synthesis method is applied to the large quantity of a material it is hard to obtain a uniform particle size distribution due to the difference of crystal growth due to a temperature gradient between the surface and the inside. Accordingly, the microwave synthesis method using the frequency of 2.45 GHz is not appropriate for an industrial mass production.